Practical investigations
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For my advanced higher investigation I am determining the vitamin C content in 3 different types of fruit juice, using titration with DCPIP and colorimetry using iodine. For the colorimetry I measured the absorbance of 8 varying concentrations of iodine solution and then plotted my graph. I mixed 25 ml of 0.003125 M iodine with 10 ml juice and tested the absorbance of this. The absorbance of the 0.003125 M iodine was 0.42 and for the juices I got the absorbances of 0.45 (from the graph iodine conc. of 0.00275), 0.50(0.003) and 0.95(0.006). I don't understand this at all please could you try and explain why this happened.
Igloo writes ..........
Your description seems to imply that your calibration curve has been derived from measuring the absorbances of solutions of aqueous iodine. You do not say what sort of juice you are using, but presumably it has some sort of colour which may be interfering with your results. Are you using a juice which is yellow/orange in colour, e.g. orange juice? If so did you use a filter which would block out this yellow/orange colour? The choice of filter is important since in very dilute solution iodine also shows up as orange, but with a different shade from that of the juice.
In summary you must use an appropriate filter to detect iodine only and this might involve some experimentation. Alternatively, repeat your calibration, but on this occasion make up your standard solutions in the juice you are using. Then your results should make more sense.
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Risk assessment
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updated: 02 April 2007
